Perhaps you use master data elements and lessons in Untis which you do not want to
export to WebUntis. You can prevent this by entering an underscore "_" for the respective element in the
"Statistical code" column.
Additionally, under <Administration> | <Integration> | <Untis>, there are additional possibilities to control the import/export between WebUntis and Untis.
The setting helps you to decide on a symbol which you can use for marking all periods you do not want to export to WebUntis in the future. Read more about this tipic here.